Carbohydrates represent the largest fraction of biomass and various strategies for their efficient use as a feedstock for the preparation of commercial chemicals are being established. Biomass is of particular interest due to its potential for supplementing, and ultimately replacing, petroleum. One such commercial chemical obtainable from biomass is lactic acid. A lactic acid derivative, methyl lactate, is a convenient building block towards renewable and biodegradable solvents and polymers.
Lactic acid derivatives, in particular esters of lactic acid, may be obtainable from sugars via a variety of reaction process routes including biochemical (enzymatic fermentation; enantiopure product), and synthetic (catalytic conversion; racemic product). Particular attention has been focused on synthetic (catalytic) routes as they provide a commercially and environmentally advantageous alternative to biochemical routes, in addition to providing a racemic product. A racemic product is advantageous if, for example, polymers that require stoichiometric amounts of both enantiomers of lactic acid enantiomers are desired as the starting materials, for example, polylactic acid (PLA).
The prior art establishes that racemic mixtures of esters of lactic acid and 2-hydroxy-3-butenoic acid may be prepared from sugars in the presence of a Lewis acid catalyst.
Esters of lactic acid and 2-hydroxy-3-butenoic acid or α-hydroxy methionine analogues may be prepared by a batch or continuous flow process from sugars in the presence of a Lewis acid catalyst. Both Science (2010) 328, pp 602-605 and EP 2 184 270 B1 disclose batch reactions for the process wherein the solvent is methanol only or methanol and 2.5 wt % water. Both references also disclose a batch reaction where the solvent is water, consequently producing lactic acid (30%) from sucrose.
In order to obtain an industrially feasible process for preparing the esters described above, it is essential that the Lewis acid catalyst remains stable, i.e. active, for a prolonged process duration. It is a well-known problem that Lewis acid catalysts deactivate over time during a reaction and must be regenerated by calcination. Deactivation of the Lewis acid catalyst requires the process to be stopped and the catalyst to be isolated and regenerated by calcination for at least 12-18 hours. Science (2010) 328, pp 602-605 and EP 2 184 270 B1 disclose that for all batch reactions the catalyst is regenerated by calcination after 20 hours.
Science (2010) 328, pp 602-605 also discloses a continuous flow process for the conversion of a sugar (fructose) to methyl lactate in the presence of a Lewis acid catalyst (Sn-BEA) and an organic solvent (methanol). FIG. 7 of the supporting data indicates that the percentage yield of methyl lactate from fructose with time on stream (TOS) is reduced by at least 50%, from about 23% at 3 hours to about 11% at 80 hours. This figure shows that Sn-BEA deactivates gradually as a function of time on stream. Similarly to the batch reactions, the catalyst is regenerated by calcination. It is noted that Sn-BEA and Sn-Beta (as used here) are identical.
Additionally, Science (2010) 328, pp 602-605 illustrates that the presence of water to the reaction process is a disadvantage with regard to catalyst stability. When the solvent of the process is only water, the carbon deposition on the catalyst is greatly increased, contributing significantly to the deactivation of the catalyst. For example, when the solvent of the process is water, 7 wt % of carbon per gram of catalyst is deposited on the catalyst, in comparison to 1.3 wt % when the process employs methanol only as the solvent.
A further example of the disadvantage of the addition of water to a process that employs a Lewis acid catalyst has been reported in Journal of Catalysis (2003) 220, pp 326-332. This reference discloses the Mukiyama-type aldol condensation of aldehydes with a silyl enol ether over a titanium silicate Lewis acid catalyst (Ti-BETA or TS-1). The reference reports that the addition of a small amount of water to the batch reaction medium during the initial reaction period decreases the activity of the catalyst. It is believed that the Lewis acid catalysts are poisoned by water and therefore become inactive. For alternative reactions, ChemSusChem (2014) 7, pp 2255-2265, reports the same effect for Sn-BEA catalysed batch reactions.
A still further example of the disadvantage of the addition of water to a process that employs a Lewis acid catalyst has been reported in Journal of Catalysis (2014) 311, pp 244-256. This reference is directed towards the study of reaction pathways of the catalytic deoxygenation of propapal (propionaldehyde). The reference discloses that Lewis acid sites of the catalyst are prevented from participating in the catalytic reaction when water is present because the water rehydrates or is physisorbed onto these sites.
It is an object of the present invention to provide a means for stabilising a Lewis acid catalyst for use in a continuous reaction process for preparing esters of lactic acid and 2-hydroxy-3-butenoic acid from a sugar. It is a further object of the present invention to provide esters of lactic acid and α-hydroxy methionine analogues from a sugar.
In addition to reducing carbon deposition on the catalyst, it is a further object of the present invention to provide a means for stabilising a Lewis acid catalyst comprising Sn, wherein leaching of Sn from the catalyst is reduced and a significantly higher yield of esters of 2-hydroxy-3-butenoic acid is obtained. The reduction in Sn leaching results in a more pure product and a cheaper process (as less Sn is required). In addition, the esters of 2-hydroxy-3-butenoic acid by-product are valuable chemicals and may provide an additional commodity from the process. More explicitly, the addition of a significantly increased yield of esters of 2-hydroxy-3-butenoic acid provide a higher combined yield of esters of lactic acid and 2-hydroxy-3butenoic acid, together with providing a higher conversion of the sugar. These advantages also apply to the preparation of the α-hydroxy methionine analogues.